A hydrogen reaction in which active hydrogen takes part, e.g., a hydrogenation reaction of organic material is employed in various chemical fields. In accordance with the cracking reaction of petroleum, for example, gasoline or kerosene can be obtained from heavy oil. Further, reaction which comprises liquefying tar content so that it is adapted more for the purpose is actually practiced. Moreover, the conversion of unsaturated hydrocarbon to saturated hydrocarbon is practiced.
Some hydrogenation reactions are often allowed to proceed in a uniform system. For example, an organic material is hydrogenated in the presence of a contact catalyst. It is known that a noble metal such as palladium is an excellent catalyst for the hydrogenation reaction of an unsaturated organic compound (S. Siegel, in "Comprehensive Organic Synthesis", ed., B. M. Trost and I. Fleming, Pergamon Press, Oxford, 1991, vol. 8). These reactions are disadvantageous in that they require a high pressure reaction vessel or normally require a relatively high temperature that can cause the explosion depending on the purity of the hydrogen gas used in hydrogenation. These reactions are also disadvantageous in that the catalyst used has an insufficient reaction selectivity and thus side reactions can occur.
In order to enhance reaction selectivity and reduce energy consumption, electrolytic reduction, which is a heterogenous system reaction, may be employed as described in A. M. Couper, D. Pletcher and F. C. Walsh, "Chem. Rev.", 1990, 90, 837, T. Nonaka, M. Takashashi and T. Fuchigami, "Bull. Chem. Soc. Jpn.", 182 56, 2584, M. A. Casadei and D. Pletcher, "Electrochim. Acta, 33, 117 (1988), T. Yamada, T. Osa and T. Matsue, "Chem. Lette.", 1989 (1987), L. Coche, B. Ehui, and J. C. Moutet, "J. Org. Chem.", 55, 5905 (1990), and J. C. Moutet, Y. Ouennoghi, A. Ourari and S. Hamar-Thibault, "Electrochim. Acta", 40, 1827 (1995). An electrode catalyst having a large surface area such as Raney nickel can be used for an electrochemical hydrogenation reaction and thus can be expected to provide a high power efficiency. Further, such an electrode catalyst provides safe and easy operation. However, this system requires that the organic material to be treated be electrically conductive. Otherwise, an additive must be added to the organic material to render the organic material electrically conductive.
As described above, hydrogenation reactions can be divided into two types, i.e., homogeneous system reactions and heterogeneous system reactions. It is known that atomic hydrogen produced on the catalyst acts to accelerate the reaction in either case.
As one of other processes for safely effecting hydrogenation reaction at a high efficiency, a process is known which comprises bringing the reaction compound to be hydrogenated into contact with palladium or other hydrogen-storing metals (metal hydride) having hydrogen held therein. It is said that palladium or many hydrogen-storing alloys also have a catalytic action in this reaction and thus can fairly act in the reaction. However, this process is disadvantageous in that once hydrogen adsorbed in the hydrogen-storing metal alloy or palladium is consumed for the reaction with a small amount of the reactant, the reaction no longer proceeds even if the remaining reactant is left unreacted. Thus, this process can be performed batchwise only. This process can be performed reasonably well on an experimental basis but at an extremely low efficiency on an industrial basis.
In order to solve these problems, the inventors proposed the following process and apparatus. In other words, electrolysis is effected in an electrolytic solution with one surface of a plate-like hydrogen-storing metal as a cathode to produce hydrogen. The hydrogen thus produced is then adsorbed by the plate-like hydrogen-storing metal at one surface thereof. The hydrogen is diffused into the hydrogen-storing metal through which it moves to-the other surface thereof. The reactant to be hydrogenated is brought into contact with the other surface of the hydrogen-storing metal at which a hydrogenation reaction or a reduction reaction by hydrogen is continuously effected. It has been obvious that this process and apparatus can find wide application in the industry and can produce a hydrogenated product at a high efficiency.
However, this reaction process is disadvantageous in that the hydrogenation reaction often is a rate-limiting step. The inventors made extensive studies of this reaction process. As a result, the following facts were found. When the current density is raised to accelerate the production of hydrogen by electrolysis, the rate of production of hydrogen exceeds the highest allowable value for hydrogenation reaction at an early stage. Even if hydrogen is present in excess, the hydrogen-storing metal can keep adsorbing and holding hydrogen. Therefore, hydrogen thus produced is rarely wasted. However, this is limited. If the current density is raised beyond a predetermined value, the current efficiency is reduced so much. In other words, this reaction process is disadvantageous in that the productivity of hydrogenated product cannot be increased beyond a certain limit.